# std::variant

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Defined in header `<variant>` |  |
template< class... Types > class variant; |  | (since C++17)

The class template `std::variant` represents a type-safe union. An instance of `std::variant` at any given time either holds a value of one of its alternative types, or in the case of error - no value (this state is hard to achieve, see `valueless_by_exception`).

As with unions, if a variant holds a value of some object type `T`, the `T` object is nested within the `variant` object itself. Variant is not allowed to allocate additional (dynamic) memory.

A variant is not permitted to hold references, arrays, or the type void.

A variant is permitted to hold the same type more than once, and to hold differently cv-qualified versions of the same type.

Consistent with the behavior of unions during aggregate initialization, a default-constructed variant holds a value of its first alternative, unless that alternative is not default-constructible (in which case the variant is not default-constructible either). The helper class `std::monostate` can be used to make such variants default-constructible.

A program that instantiates the definition of `std::variant` with no template arguments is ill-formed. std::variant<std::monostate> can be used instead.

If a program declares an explicit or partial specialization of `std::variant`, the program is ill-formed, no diagnostic required.

### Template parameters

Types | - | the types that may be stored in this variant. All types must meet the Destructible requirements (in particular, array types and non-object types are not allowed).

### Member functions

(constructor) | constructs the `variant` object (public member function)
(destructor) | destroys the `variant`, along with its contained value (public member function)
operator= | assigns a `variant` (public member function)
##### Observers
index | returns the zero-based index of the alternative held by the `variant` (public member function)
valueless_by_exception | checks if the `variant` is in the invalid state (public member function)
##### Modifiers
emplace | constructs a value in the `variant`, in place (public member function)
swap | swaps with another `variant` (public member function)
##### Visitation
visit (C++26) | calls the provided functor with the argument held by the `variant` (public member function)

### Non-member functions

visit (C++17) | calls the provided functor with the arguments held by one or more `variant`s (function template)
holds_alternative (C++17) | checks if a `variant` currently holds a given type (function template)
get(std::variant) (C++17) | reads the value of the variant given the index or the type (if the type is unique), throws on error (function template)
get_if (C++17) | obtains a pointer to the value of a pointed-to `variant` given the index or the type (if unique), returns null on error (function template)
operator==operator!=operator<operator<=operator>operator>=operator<=> (C++17)(C++17)(C++17)(C++17)(C++17)(C++17)(C++20) | compares `variant` objects as their contained values (function template)
std::swap(std::variant) (C++17) | specializes the std::swap algorithm (function template)

### Helper classes

monostate (C++17) | placeholder type for use as the first alternative in a `variant` of non-default-constructible types (class)
bad_variant_access (C++17) | exception thrown on invalid accesses to the value of a `variant` (class)
variant_sizevariant_size_v (C++17) | obtains the size of the `variant`'s list of alternatives at compile time (class template) (variable template)
variant_alternativevariant_alternative_t (C++17) | obtains the type of the alternative specified by its index, at compile time (class template) (alias template)
std::hash<std::variant> (C++17) | hash support for `std::variant` (class template specialization)

### Helper objects

variant_npos (C++17) | index of the `variant` in the invalid state (constant)

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_variant` | `201606L` | (C++17) | `std::variant`: a type-safe union
`202102L` | (C++23) (DR17) | std::visit for classes derived from `std::variant`
`202106L` | (C++23) (DR20) | Fully `constexpr` `std::variant`
`202306L` | (C++26) | Member `visit`

### Example

Run this code

    #include <cassert>
    #include <iostream>
    #include <string>
    #include <variant>

    int main()
    {
        std::variant<int, float> v, w;
        v = 42; // v contains int
        int i = std::get<int>(v);
        assert(42 == i); // succeeds
        w = std::get<int>(v);
        w = std::get<0>(v); // same effect as the previous line
        w = v; // same effect as the previous line

    //  std::get<double>(v); // error: no double in [int, float]
    //  std::get<3>(v);      // error: valid index values are 0 and 1

        try
        {
            std::get<float>(w); // w contains int, not float: will throw
        }
        catch (const std::bad_variant_access& ex)
        {
            std::cout << ex.what() << '\n';
        }

        using namespace std::literals;

        std::variant<std::string> x("abc");
        // converting constructors work when unambiguous
        x = "def"; // converting assignment also works when unambiguous

        std::variant<std::string, void const*> y("abc");
        // casts to void const* when passed a char const*
        assert(std::holds_alternative<void const*>(y)); // succeeds
        y = "xyz"s;
        assert(std::holds_alternative<std::string>(y)); // succeeds
    }

Possible output:

    std::get: wrong index for variant

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2901 | C++17 | specialization of std::uses_allocator provided, but `std::variant` cannot properly support allocators | specialization removed
LWG 3990 | C++17 | a program could declare an explicit or partial specialization of `std::variant` | the program is ill-formed in this case (no diagnostic required)

### See also

in_placein_place_typein_place_indexin_place_tin_place_type_tin_place_index_t (C++17) | in-place construction tag (tag)
optional (C++17) | a wrapper that may or may not hold an object (class template)
any (C++17) | objects that hold instances of any CopyConstructible type (class)

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